pulseaudio: add target option to control source (microphone) volume

The pulseaudio/slider module had partial support for a "target" config
option but changeVolume() in the audio backend was hardcoded to only
modify sink volume. This adds full source volume control:

- Add AudioTarget enum (Sink/Source) to audio_backend
- Extend changeVolume() overloads with AudioTarget parameter
- Store and use pa_cvolume for source (with mono channel default)
- Add sourceVolumeModifyCb for source volume change confirmation
- Add "target" config option to the pulseaudio module for scroll control
- Document "target" in both pulseaudio and pulseaudio-slider man pages

Usage:
  "pulseaudio/slider#in": { "target": "source" }
  "pulseaudio#in": { "target": "source" }

include/modules/pulseaudio.hpp

@@ -22,6 +22,7 @@ class Pulseaudio : public ALabel {
   const std::vector<std::string> getPulseIcon() const;
 
   std::shared_ptr<util::AudioBackend> backend = nullptr;
+  util::AudioTarget target = util::AudioTarget::Sink;
 };
 
 }  // namespace waybar::modules